Plasma reactors, for example as used in fabricating semiconductor integrated circuits, can present extreme demands upon the materials constituting the chamber walls and other components within the reactor that are exposed to the plasma. A particularly difficult environment is presented by an oxide etcher used in etching through layers of silicon dioxide in semiconductor integrated circuits.
Such a reactor is shown in schematic cross section in FIG. 1 and closely follows the Centura HDP Oxide Etcher, available from Applied Materials, Inc. of Santa Clara, Calif. This reactor is exemplary only and is described here to provide an understanding of the operation of a plasma reactor. The reactor 10 includes a vacuum chamber 12 into which is loaded a silicon wafer 14 that is supported on a pedestal 15. In the illustrated reactor, the wafer 14 is held by a plasma focus ring 16 and a clamping ring 18, and an annular plasma guard 20 surrounds the pedestal 15 to protect it from the plasma. If an electrostatic chuck is used in the pedestal 15, it may be surrounded by an unillustrated free standing collar which controls the plasma conditions in the neighborhood of the wafer 14. This description is intended to be illustrative only and not defining the preferred configuration.
The vacuum chamber 12 includes a cylindrical dielectric wall 22, outside of which is wrapped an inductive coil 24 for coupling RF energy into the chamber 12 so as to create therein a high-density plasma of processing gases admitted into the chamber 12 by unillustrated gas ports. A roof 26 defines the upper boundary of the chamber 12. The roof 26 is often grounded or even biased by an RF electrical source and thus preferably functions as a counter electrode. Alternatively or additionally, the roof 26 is preferably temperature controlled by a heater/cooler 30, which also serves as the vacuum-sealing roof, and may be electrically connected thereto so as to be commonly biased or grounded. The foregoing reactor structure is being given only by way of example so as to explain the usefulness of the invention, and the invention is equally applicable to other reactor structures and is not limited to oxide etchers.
In oxide etching, a fluorocarbon plasma, for instance of CF.sub.4 or C.sub.3 F.sub.8 among other examples, is used to etch through an oxide layer, typically silicon oxide overlying silicon or polysilicon. To assure uniform etching, it is important that the etching process strongly preferentially etch the oxide over the silicon. In the normal parlance, the etch should be selective to silicon.
The plasma is largely supported by the inductive coil 24, which couples a large amount of RF energy into the chamber 12 and thus generates a high-density plasma (HDP), which allows a very high etching rate of the oxide layer on the wafer 14.
However, obtaining selectivity, uniformity of selectivity over the wafer, and reproducibility of selectivity over slightly varying conditions has provided technological challenges in commercializing HDP oxide etchers. It was early recognized, as Collins et al. have described in European Patent Publication 552,491-A1, that a chamber element such as the roof 26 be formed of silicon. Under the proper conditions, the solid silicon provides species of silicon which combine with fluorine radicals in the plasma so that a fluorine-deficient carbonaceous polymer forms on the exposed elemental silicon surfaces, thereby enhancing the oxide-over-silicon selectivity. It was however also recognized that the temperature of the chamber silicon including the silicon roof 26 was important in controlling the process and that proper temperature control means for heating as well as cooling the chamber silicon should be incorporated into an oxide etcher, for example, the heater/cooler 30. It was also recognized that control of electrical biasing of the roof 26 could likewise be used to control the process.
Thereafter, attention passed to the wall 22 inside the inductive coil 24, which has been typically formed of quartz, a crystalline form of silicon oxide. Rice et al. disclose in U.S. Pat. No. 5,477,975 regulating the wall's temperature relative to that of the roof 26 to control the selectivity. It has been further recognized that other parts surrounding the wafer 14 need to have their temperature controlled to control the process. Such parts, which hitherto have been typically made of quartz, include the clamping ring 16, the plasma guard 18, and the plasma ring 20.
However, the temperature control of quartz parts is difficult. Quartz is both an electrical and thermal insulator. Its coefficient of thermal conductivity is less than 1 W/m.multidot.K, a relatively low value, so that it is difficult to closely control the temperature of the entire surface of a quartz piece exposed to the plasma. Furthermore, quartz has a chemical composition closely resembling that of the silicon oxide layer being etched in the semiconductor oxide etch reactor. Thus, one must assure that the quartz part is being operated in a deposition or slow etch mode rather than a strong etch mode while the wafer of similar composition is being strongly etched. If not, the quartz parts will have short lifetimes and hence impose a high cost of replacing consumable parts, both in terms of parts cost and machine down time. Furthermore, the quartz, although it is a silicate material providing some silicon scavenging, also produces a relatively uncontrollable amount of CO and CO.sub.2 from the reaction between, for example, CF.sub.4 and SiO.sub.2. The resultant carbon monoxide and dioxide are particular problems for selectivity to photoresist. Also, quartz is a ceramically formed material and typically includes large amounts of non-silicate components, which become contaminants in the fluorocarbon etching environment. Even further, the etching of the quartz can undercut surface portions to the extent that particles of quartz are separated from the reactor elements and fall onto the wafer as fatal particles. Oxide etching is particularly critical against particles since the etching produces interfaces between two electrically conducting parts, one of which may be semiconducting and any particle falling on the interface before deposition of the subsequent layer can seriously affect the electrical characteristics of the junction across the interface.
Collins et al. have suggested in European Patent Application 601,468-A1 and in U.S. patent application, Ser. No. 08/597,577, filed Feb. 2, 1996 that an inductive coil be placed in back of the silicon roof 26. Further, in the latter, Collins et al. have suggested that other parts of the chamber, including the side walls 22 in front of the RF coils 24 be formed of silicon, either in its crystalline or polysilicon structure. The silicon composition provides some scavenging functions and also avoids contamination by quartz or other ceramics. However, silicon is a semiconductor, not a dielectric. As described by Collins et al., the silicon of the proper doping and thickness can advantageously also be electrically biased, either DC or RF, even while, in a preferred usage, electromagnetic radiation is being propagated therethrough. However, silicon in such uses presents many compromises and disadvantages. First, silicon in such large dimensions is not readily available at reasonable prices, particularly in times of shortage of polysilicon. Secondly, silicon, although affording relatively high structural strength, is prone to fracture from local micro-defects arising from its growth in the form of polysilicon and its subsequent machining. Thirdly, semiconducting silicon (bandgap of about 1.2 eV) affords an uneasy compromise between structural strength and electromagnetic transparency. Electromagnetic radiation can only penetrate a semiconductor or other conductor to the extent of a skin depth which can be expressed as ##EQU1## where f is the frequency of the electromagnetic radiation in hertz, .mu..sub.0 is the magnetic permeability in H/m, and .rho. is the bulk DC plasma resistivity of the semiconductor in ohm-m. The penetration of electromagnetic radiation through a conductive sheet is generally an exponential function of the sheet thickness z having the general form to first order of where surface effects are disregarded. These relationships show that the transparency of a layer depends on both the material resistivity and the frequency of the electromagnetic radiation. The functional dependence (2) shows that for a thickness of no more than the skin depth, the resistive absorption is less than 64%; for a thickness of no more than one-third the skin depth, the absorption is less than 29%; and for a thickness of no more than one-tenth the skin depth, the absorption is less than 10%.
The skin depth for available semiconductor materials at reasonable operating temperatures may be considerably less than a realistic structural thickness, even for vacuum wall members. A typical inductively coupled plasma reactor uses an RF source with a frequency of 2 MHz. For this frequency and for a non-magnetic material, the estimated skin depths for representative material resistivities are given in TABLE 1
TABLE 1 ______________________________________ Resistivity Skin Depth (ohm-cm) (mm) ______________________________________ 0.1 1.13 0.3 1.95 1 3.56 3 6.17 10 11.3 30 19.5 100 35.6 ______________________________________
Many have recognized that plasma reactor chamber parts can be formed of silicon carbide, both for its high-temperature performance and for its fluorine-scavenging characteristics. However, we observe that bulk silicon carbide, at least in its sintered or hot-pressed forms, is inadequate at least in a fluorine etching environment if few particles and long part life are to be attained and if high chemical purity is required in the wafer processing chamber. Silicon carbide is sintered by mixing a silicon carbide powder with a generally pliable sintering aid. The sintering aid has a complex and ill-defined composition producing effective impurity levels on the order of hundreds of parts per million, which greatly exceed the parts-per-billion scale required for wafer processing chambers. Also, the sintering aid, even after it has been hardened in the sintering process, produces a highly granular structure that exhibits a strongly differential pattern of etching, thus becoming a mechanism for producing particulates. In contrast, in CVD SiC, the impurity levels can be controlled, if required, down to less than 100 ppb (parts per billion). Further, our experiments show that CVD SiC demonstrate uniform etch profile when being etched by a fluorocarbon etchant.
Silicon carbide is well known as a susceptor material for RF induction heating of a chamber or wafer support within the chamber. Its thermal conductivity is in the range of 100 to 200 W/m.multidot.K, vastly superior to quartz. An RF coil wrapped around the chamber induces eddy currents in a highly conductive silicon carbide part to thereby heat it to high temperatures, such as are required for thermal CVD, as disclosed by Ban in U.S. Pat. No. 4,401,689. Induction heating is to be contrasted with inductive coupling of RF power into a plasma reactor chamber. Plasma reaction chambers are usually operated at much lower temperatures, and the RF energy should be coupled into the plasma and not into chamber parts. Hence, any silicon carbide parts used in a reaction chamber, at least in the vicinity of the coils, should have relatively high electrical resistivity, for example, above 10.sup.4 ohm-cm.
Silicon carbide composites are well known in which a bulk piece of silicon carbide is coated with a thin film of silicon carbide using a chemical vapor deposition (CVD) or similar process. Such composites include resistive heaters, as disclosed by Ito et al. in U.S. Pat. No. 4,810,526, and rugged mirrors, as disclosed by Hotate et al. in U.S. Pat. No. 5,448,418. Matsumoto et al. in U.S. Pat. No. 4,999,288 discloses using a silicon carbide composite as a diffusion tube for heat treating semiconductor wafers at about 1200.degree. C. According to Matsumoto et al., a 500 .mu.m-thick silicon carbide film is CVD deposited on the interior of a reaction sintered silicon carbide tube, and the film has a low concentration of iron impurities, although this level is defined as 5 parts per million (ppm). If desired, an interfacial region in the silicon carbide tube can be depleted of silicon. Electrical resistivity is immaterial in most diffusion tubes.